This invention relates to high voltage distribution circuit oil expulsion fuses in general and more particularly, it is concerned with a fuse of this variety which as compared with a single bore fuse of the same element material and cross sectional area has greater ampacity as well as improved arc-quenching capability, greater ability to interrupt against high rates of rise of transient recovery voltage and to withstand the normal frequency recovery voltage indefinitely without arcing restrikes.
Oil expulsion fuses have long been used in distribution circuits to protect electrical equipment from the deleterious effects of fault currents. Such fuses typically comprise a tubular fuse cartridge having conductive terminal caps at opposed ends, and adapted to receive a single expendable fuse link comprising an elongate fusible element contained within one arc tube. When an overcurrent or fault current is experienced in the circuit containing the fuse, the fusible element is caused to melt, whereupon arcing occurs within the arc tube between the severed segments of the fusible element. Heat produced by the arc vaporizes oil filling the tube producing pressurized deionizing gas therefrom which vents at opposite ends of the fuse cartridge. As the oil derived venting gases flow past the arc, they serve to cool and deionize the latter such that the arc is effectively extinguished. The disabled fuse may subsequently be returned to service by simply replacing the expendable fuse link contained within the fuse cartridge.
While oil expulsion fuses have heretofore generally proven satisfactory for use in distribution circuits, it has been found that there are some difficulties associated with their use in high voltage applications. With single bore oil expulsion fuses, problems have been encountered in attempting to interrupt a low fault current against a high rate of rise recovery voltage. This difficulty occurs over a wide range of application voltages. In this regard, it has been discovered that when conventional oil expulsion fuses are constructed to provide the required current capacity for use in the higher distribution voltage circuits, the fuses do not reliably interrupt against higher transient recovery voltage rates associated with the higher distribution voltages. Of course, this is a highly undesirable characteristic since it represents a failure to clear the overcurrent and could result in serious damage to electrical apparatus relying upon the fuse for current protection. While it has not been conclusively determined why larger conventional oil expulsion fuses are not adapted for use in high voltage distribution circuits, one theory is that the larger internal diameter of the arcing tube required to accomodate the desired ampacity fuse element, precludes development of deionizing gas flow sufficient to adequately extinguish the arc against the higher transient recovery voltages. In any event, there simply is not commercially available a high ampacity refusible oil expulsion fuse capable of reliably interrupting a wide range of fault currents or harmful overcurrents. It has now been discovered that these interruption and restrike problems can be avoided by using a multiple fusible element design with each fusible component received within a separate relatively small diameter fuse bore and wherein gases generated during arcing are controlled to assure interruption without restrike.
While certain types of multiple fusible element expulsion fuse units have been proposed, such as those shown in U.S. Pat. Nos. 2,156,058 to Lohausen and 2,291,341 to Lincks, these have of necessity been restricted to use in air. These air environment devices have no practical application in connection with fusing in oil because they are incapable of allowing connective flow of oil therethrough. Both of the patents identified show an expulsion fuse having a plurality of fusible elements, each element being provided with its own arcing chamber. While these fuses may exhibit limited increased current-carrying capacity for air applications, it is manifest that neither can be used in an oil system. The apparent difficulty in Lohausen's fuse of clearing all of the tubes of residual metal even though arcing occurred in only one or a limited number of bores upon interruption was recognized by Linck who sought to solve the problem with mechanical contrivances in the nature of ejector springs for the fusible elements of each bore. Even this device though would have a limited unfavorably retarded response time with the arc being retained for an undesirable period especially during low current faults.